The mobility of transtibial, or below knee, amputees is limited by the design of common, passive ankle-foot prostheses. The muscles that span the biological ankle joint provide the majority of mechanical power during walking. Consequently, the loss of this joint causes these individuals to walk up to 40% slower and expend at least 20% more metabolic energy, when compared to non-amputees; a metabolic burden typically associated with carrying 15 kg.
Historically, energy storage and release (ESR) prosthetic feet have been developed to reduce some of the aforementioned deficits associated with transtibial amputations. These prosthetic feet typically consist of an anthropomorphic carbon fiber leaf spring, where the leaf spring is cantilevered from the heel anteriorly towards the toe. The purpose of the leaf spring is to store elastic energy as the foot is dorsiflexed during the stance phase of walking, the region of the gait cycle when weight is borne by the leg. Ideally, this energy is returned to the wearer at terminal stance phase, when the ankle plantarflexes to propel the wearer forward.
As a result of the cantilever nature of the leaf spring in the design of ESR prosthetic feet, they do not provide the biologically appropriate torque-angle and stiffness properties during walking. That is, as the center of pressure moves anteriorly during stance phase, the stiffness of the cantilever leaf spring decreases exponentially. This opposes the trend known to occur in the biological ankle joint. The stiffness of the biological ankle joint is known to increase linearly during the dorsiflexion region of stance phase. Therefore, the anthropomorphic cantilever design of ESR feet results in non-biological mechanical behavior, likely contributing to the gait deficits of transtibial amputees.
Researchers have previously designed novel passive and quasi-passive prosthetic feet to address the limitations of current technology. Hansen and Nickel designed an ankle-foot prosthesis to increase balance during walking and standing. To this end, their device incorporated a locking mechanism to transition between two stiffness modes to provide the appropriate kinematic rollover shape observed during walking and standing. Additionally, Collins and Kuo developed a quasi-passive ankle prosthesis that recycles the impact energy from heel contact and returns it during push off. This prosthesis technology was shown to increase ankle push off work and decrease the metabolic energy consumed during walking, when compared to walking with a conventional prostheses. Lastly, recently developed powered ankle prostheses have been shown to normalize transtibial amputee gait characteristics.
Previous work in the development of novel ankle-foot prostheses is encouraging and underscores the importance of prosthesis stiffness properties, as well as the significance of energy returned to the wearer during locomotion. Unfortunately the clinical impact of such work has been limited by mechanical complexity; non-biological, cantilever stiffness behavior; as well as substantial mass.
By design, commonly worn energy storage and release (ESR) prosthetic feet cannot provide biologically realistic ankle joint torque and angle profiles during walking. Additionally, their anthropomorphic, cantilever architecture causes their mechanical stiffness to decrease throughout the stance phase of walking, opposing the known trend of the biological ankle.